The present invention relates generally to isolation-providing voltage converters having isolated input and output grounds, and more particularly to such voltage converters with isolated input and output grounds that employ an output-side pulse width modulator controller.
Voltage converters receive an input voltage (Vin) that is AC in an AC:DC power converter, or DC in a DC:DC power supply, and generate one or more output voltages (Vo1, Vo2) therefrom. The output voltages may be greater than Vin or less than Vin, and may be AC or DC although commonly the converter output voltage Vo1, Vo2 will be rectified. However, as used herein, in the broadest sense xe2x80x9cvoltage converterxe2x80x9d can include an AC:AC converter, an AC:DC converter, a DC:DC converter, or a DC:AC converter, wherein each of the converter types provides isolation between input-side ground and output-side ground, and may be implemented using different topologies.
FIG. 1A depicts a prior art isolation-providing voltage converter, here a DC:DC power converter 10 that converts an input voltage (Vin) to a rectified DC output voltage Vo1. A load (not shown) will be coupled between Vo1 and output ground. Although FIG. 1A actually depicts generation of a single output Vo1, it is understood that more than one output voltage could be generated, and that each such output voltage could be of a different magnitude. System 10 could be an AC:DC power converter, in which case Vin would represent an input AC voltage after it has been rectified for presentation to converter 10. System 10 could also be an AC:AC power converter or a DC:AC converter, in which case the output side lowpass filtering components may be omitted.
Converter 10 provides isolation in that there is an input side ground and an output side ground, with isolation between the two grounds. Converter 10 may thus be said to have a primary or input side 20 that receives operating potential Vin relative to an input-side ground. Converter 10 also has a secondary or output side 30 that outputs potential Vo1 relative to a secondary-side ground.
In the exemplary topology of FIG. 1A, isolation between input side 20 and output side 30 is maintained by transformer T1 and by an isolation mechanism I1. Transformer T1 typically comprises at least one primary winding W1 and at least one secondary winding W2, etc. from which raw output voltage is provided. Isolation mechanism I1 may include optical transmitter-receiver pairs, sampled signal, transformed-coupled circuits and the like.
The input side of converter 10 includes a switch Q1 coupled in series between an end of a primary transformer winding and input-side ground (or other input-side reference potential). While FIG. 1A depicts switch Q1 coupled in series between W1 and ground, it is understood that the roles of Vin and ground could be reversed, e.g., Q1 could instead be coupled between the Vin node and winding W1. If additional primary side windings are present, each such winding will have a switch such as Q1, also coupled between an end of the winding and input side ground (or other input-side reference potential, perhaps Vin).
In a fashion well known to those skilled in the relevant art, each switch Q1 opens and closes in response to a drive signal from a drive circuit 50. Drive circuit 50 functions in response to input from a pulse width modulator (PWM) circuit 60, which itself operates preferably in response to feedback 70 from the generated output-side voltage(s), here Vo1. Typically the output side of system 10 will compare Vo1 to a stable reference voltage derived from Vo1 and generate a correction signal to PWM 60, which correction signal is represented by feedback loop 70. When circuit 50 outputs a drive signal causing Q1 to turn-on, Q1 closes and Vin is impressed across the input or primary transformer winding W1, and essentially Vin is sampled or chopped. The resultant chopped signal is inductively coupled via transformer T1 to the secondary transformer winding W2.
With the specific topology shown in FIG. 1A, on the output side, diode D1 and lowpass filter L1-C1 rectify and filter the signal to yield an output DC voltage, Vo1. (Of course other output side topologies and/or rectification configurations could instead be used.)
The magnitude of Vo1 may be altered by changing duty cycle of the drive signal provided by circuit 50 to switch Q1, which is to say by pulse width modulating the drive signal output from circuit 50. In the configuration shown, drive signal PWM changes are responsive to a signal or signals from PWM 60 in response to a feedback signal via feedback path 70. As a result, circuit 50 can make compensating changes in the drive signal delivered to the input of switch Q1. For example, if the load or other factors cause Vo1 to decrease, feedback via path 70 can cause PWM circuit 60 to drive circuit 50 to increase duty cycle of the drive signal to switch Q1 to increase magnitude of Vo1.
In FIG. 1A, driver circuit 50 and PWM circuit 60 are referenced to the input side of converter 10, which is to say these circuits are directly coupled to input-side ground. A practical consideration for circuit 50 and PWM circuit 60 is establishing a bias operating potential, Vbias, to ensure that these circuits can operate as soon as Vin is provided to converter 10. For the input side configuration shown in FIG. 1A, providing bias voltage is straightforward. Among other techniques, Vbias may be directly derived from Vin, for example using a circuit 40 comprising Zener diode Vz, current-limiting resistor R1, and filter capacitor C1. Another approach is to obtain a bias potential from a primary winding on T1, which Vbias approach is suggested by a phantom line in FIG. 1A. Generating input-side Vbias is relatively straightforward for the topology of FIG. 1 because Vin, drive circuit 50, and PWM 60 are each referenced to input-side ground.
But although providing an input-side referenced control circuit 50 and PWM circuit 60 enables a simplified Vbias biasing circuit to be used, it is necessary to include an isolation mechanism I1 to isolate the output-side ground signals from the input-side grounded PWM circuit 60. In addition to adding implementation cost and bulk to system 10, isolation components (e.g., optical transmitter-receiver pairs, transformed-coupled circuits and the like) tend to reduce useful feedback bandwidth. By way of example, optical transmitter-receiver pairs used for I1 tend to limit feedback bandwidth of loop 70 to perhaps 5 KHz. Understandably large feedback bandwidth is desired to ensure a more rapid correction of Vo1, preferably at least 20 KHz.
System 10 in FIG. 1B is somewhat similar to what was shown in FIG. 1A except that pulse width modulation circuit 60 is now referenced to the output-side of voltage converter 10. In this configuration, advantageously no isolation components are required between Vo1 and the input to PWM circuit 60 as the PWM circuit is also now referenced to output-side ground. The absence of isolating elements between Vo1, and PWM 60 can maintain a high feedback bandwidth. Unfortunately, however, it is now necessary to provide isolation I1, I2 between the output of PWM circuit 60 and the input to drive circuit 50, since the input components are referenced to input-side ground. As a result, providing Vbias to PWM circuit 60 is complicated by the necessity to include isolation, shown here as I2. Isolation I2 typically is implemented with a chopper switch (indicated as Q2) and an AC-coupled isolating transformer and output-side rectifier. Isolation I1 may be similarly implemented, or may instead use optical isolating devices. A practical consideration in implementing I2 is the necessity to comply with U.L. isolation requirements, as I2 spans between the input-side ground and the output-side ground portions of voltage converter system 10.
Thus in an isolating or isolation-providing voltage converter, although it is advantageous to use an output-side referenced PWM circuit 60 to maintain high feedback gain and ease of output-side connections, such configurations complicate generating Vbias potential to ensure that circuit PWM 60 will function as soon as Vin is applied. Also, as shown in FIG. 1B, an output-side referenced PWM circuit 60 requires isolation I1 to provide a drive signal to the input-side driver circuit 50, whose output drives switch Q1. As I1 does not deliver substantial power, I1 typically can be implemented with optical transmitter-receiver pairs, sampled signal, transformed-coupled circuits and the like.
There is a need for a voltage converter topology that advantageously provides the high gain and relative simplicity of output-side referenced control and pulse width modulation circuitry, while overcoming the problems in the prior art associated with generating Vbias potential for such circuitry.
The present invention provides such topology and a method of implementation.
The present invention provides a Vbias voltage generator to provide at least initial operating potential for an output-ground reference pulse width modulator (PWM) circuit in a voltage converter. The Vbias generator operates similarly to a charge pump and includes a waveform generator that operates from the converter""s Vin input potential and outputs a periodic signal. The periodic signal may be a pulse train, a sinusoid, or other periodic waveform. The periodic signal is preferably input to an input-side referenced circuit that can amplify and output the signal with a complementary version of the signal. At this juncture, the periodic signal may be single-ended or differential. The signal from the circuit is AC-coupled through isolating capacitors to a rectifier circuit that is referenced to output-side ground and whose output is Vbias potential. The rectified Vbias potential at the output-side is coupled to the output-side PWM as the start-up Vbias potential, to ensure the PWM operates properly from application of Vin potential to the converter.
A more robust source of Vbias for the PWM may be generated from a secondary winding on the converter transformer. This potential is rectified and is also applied to the Vbias node of the PWM. During the initial perhaps 10 ms of application of Vin (e.g., from power-up) to the converter, the Vbias potential is provided by the Vbias generator. After that time duration the PWM will be operating normally and Vbias may be then obtained from the extra winding on the power transformer. If desired, the Vbias generator can be turned-off once the voltage converter reaches steady-state, e.g., after about 10 ms.
The output-side referenced PWM obtains feedback information directly from the voltage converter output(s) and can thus preserve high loop bandwidth. The output from the PWM may be coupled to a conventional drive circuit using a prior art isolator, for example a transformer circuit.
Other features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail, in conjunction with the accompanying drawings.